A number of anti-cancer drugs are currently in clinical use for the treatment of various cancers. For example, paclitaxel and taxotere are two promising anti-cancer drugs used to treat breast and ovarian cancers, and which hold promise for the treatment of other cancers such as skin, lung, head and neck carcinomas. Other promising chemotherapeutic agents are being developed or tested for treatment of these and other cancers. Compounds such as paclitaxel, taxotere, and other taxanes, camptothecins, epothilones and quassinoids, as well as other compounds exhibiting efficacy in cancer treatment, are of considerable interest. Of special interest are natural product drugs with demonstrated anticancer activity, in vitro and in vivo. Such compounds are desirable, for example, for their potential availability from renewable resources.
However, many identified anti-cancer compounds present a number of difficulties with their use in chemotherapeutic regimens. One particular problem relates to the aqueous insolubility of many anti-cancer compounds, which creates significant problems in developing suitable pharmaceutical formulations useful for chemotherapy. In an attempt to increase the aqueous solubility of these drugs, they are often formulated with various carrier compounds. However, these carrier compounds often cause various adverse side effects in a patient treated with the formulation. For example, paclitaxel and camptothecin and their analogs are generally formulated with a mixture of polyethoxylated castor oil (Cremophore) and ethanol. This mixture has been reported to cause side effects in clinical trials, which include neutropenia, mucositis, cardiac and neurological toxicities, hypersensitivity, histamine release and severe allergic reactions.
Another problem with the use of such chemotherapeutic agents in cancer treatment is the difficulty targeting cancer cells without adversely affecting normal, healthy cells. For example, paclitaxel exerts its antitumor activity by interrupting mitosis and the cell division process, which occurs more frequently in cancer cells than in normal cells. Nonetheless, a patient undergoing chemotherapy treatment may experience various adverse side effects associated with the interruption of mitosis in normal, healthy cells.
Accordingly, it would be highly desirable to develop chemical compounds and methods for use in directly targeting cancer cells with chemotherapeutic agents in cancer treatment regimens. This, in turn, could lead to reduction or elimination of toxic side effects from carrier compounds, more efficient delivery of the drug to the targeted site, and reduction in dosage of the administered drug and a resulting decrease in toxicity to healthy cells and in the cost of administering the chemotherapeutic regimen.
One particular approach of interest is the use of anti-cancer drug moieties that have been conjugated to tumor-recognizing molecules. For example, U.S. Pat. No. 6,191,290 to Safavy discusses the formation and use of a taxane moiety conjugated to a receptor ligand peptide capable of binding to tumor cell surface receptors. Safavy in particular indicates that such receptor ligand peptides might be BBN/GRP receptor-recognizing peptide, a somatostatin receptor-recognizing peptide, an epidermal growth factor receptor-recognizing peptide, a monoclonal antibody or a receptor-recognizing carbohydrate.
One tumor-recognizing molecule of particular interest is the human protein Transferrin. Transferrin is a serum glycoprotein of approximately 79550 molecular weight, which is involved in iron transport to developing red cells for hemoglobin synthesis. It has a very high binding affinity for ferric iron so that essentially no free ferric iron, a very toxic form of iron, occurs in plasma. Further, the iron requirement of growing cells is provided by diferric Transferrin (each protein molecule specifically binds with two Fe3+ ions to form salmon-pink complexes) which binds to receptors on the cell membrane leading to an internalization of the Transferrin-receptor complex which then leads to a release of iron to the cytoplasm of the cell and return of the apoTransferrin-receptor complex to the cell surface and release of the apoTransferrin from the receptor. It has been demonstrated that growing cells have Transferrin receptors on their cell surface whereas static cells either do not or have very low numbers of Transferrin receptors. Further, cancer cells have been demonstrated to have a high number of Transferrin receptors and interestingly, drug resistant cancer cells have an even greater number of Transferrin receptors. The presence of Transferrin receptors on cancer cells but not on normal cells suggests that Transferrin conjugates could provide a selective way of targeting agents to cancer cells. For instance, as reported by Yeh et al., “Killing of Human Tumor Cells in Culture with Adriamycin Conjugates of Human Transferrin”, Clin. Immunol. Immunopathol. 32, 1–11 (1984), and by Sizensky et al., “Characterization of the Anti-Cancer Activity of Transferrin-Adriamycin Conjugates”, Am. J. Reprod. Immunol. 27:163–166 (1992), Transferrin-adriamycin conjugates have a higher therapeutic index than free adriamycin for cancer therapy.
Other works suggest a promising approach to cancer therapies utilizing Transferrin conjugated with various chemotherapeutic drugs, such as Doxorubicin (Kratz et al., “Transferrin conjugates of Docorubicin: Synthesis, Characterization, Cellular Uptake, and in Vitro Efficacy”, J. Pharm Sci., 87, 338–346 (1998)) and Mytomycin C (Tanaka et al., “Synthesis of Transferrin-Mitomycin C Conjugate as a Receptor-Mediated Drug Targeting System”, Biol. Pharm. Bull. 19, 774–777 (1996)).
An attempt at an effective Transferrin-paclitaxel conjugate was reported by Bicamumpaka et al., “In Vitro Cytotoxicity of Paclitaxel-Transferrin Conjugate on H69 Cells”, Oncol. Rep., 5, 1381–1383 (1998). In particular, Bicamumpaka et al. synthesized 2′-glutaryl-hexanediamine paclitaxel, which was then coupled to Transferrin using a glutaraldehyde linker through an amino of the 2′-glutaryl-hexanediamine group. However, Bicamumpaka reported that the capacity of the resulting Transferrin-paclitaxel conjugate to inhibit growth of H69 cells was 5.4 times less than that of the native paclitaxel drug.
Accordingly, it can be seen that there is a need to provide new chemical compounds for linking chemotherapeutic agents to various molecules, such as Transferrin, the receptor ligand peptides recognized by Safavy, or other proteins, antibodies, lectins or other substances that may become attached to the surface of a cell. There is also a need to provide methods for forming such compounds. It can further be seen that there is a need for new molecular conjugates for use in treating cancer, and Transferrin-drug conjugates in particular. Finally, there is a need for new methods of administering chemotherapeutic pharmaceutical formulations to patients for use in cancer treatment regimens, such as through the use of improved molecular conjugates such as Transferrin-drug conjugates. The present invention is directed to meeting these needs.